The light source of the PHELIX beamline is an elliptically polarizing undulator with permanent magnets EPU APPLE II type. Due to the possibility of moving two opposite jaws of the undulator along the electron beam, linear and circular or elliptical polarizations of light are realized (fig. 1).
EPU parameters:
- Total length of the magnets arrays: 2600 mm
- Period length: 58 mm
- Number of periods: 44
- Minimal gap: 14 mm
Fig. 1. Diagram showing the method of producing light of different polarization by changing the phase of the undulator.
Fig. 2. Photon flux per second emitted by the source for different light polarizations and measured before the last focusing mirror for a diffraction grating of 600 l/mm, opening of the moving masks of 3 mm x 3 mm, exit slits of 50 µm and cff = 2.25.
The optical system of the PHELIX beamline consists of a collimating mirror (M1), a plane mirror (M2) with three gratings (G1, G2, G3) installed in the monochromator chamber, a focusing switching mirror (M3) and a focusing mirror installed before the end-station (M4) (fig. 3). Table 1 presents the parameters of optical elements of the PHELIX beamline.
Fig. 3. Model showing the optical system of the PHELIX beamline.
Table 1. Parameters of optical elements installed on the PHELIX beamline.
| M1 |
M2 (PGM) |
G1 (PGM) |
G2 (PGM) |
G3 (PGM) |
M3 | M4 | |
|---|---|---|---|---|---|---|---|
| Shape | cylinder | plane | plane | plane | toroid | toroid | |
| Cooling | intrinsic | side | side | side | side | no | no |
| Energy range [eV] | full range | full range | 50 - 300 | 200 - 1000 | 400 - 1500 | full range | full range |
| Dimensions [mm2] | 370x60 | 470x50 | 170x30 | 170x30 | 170x30 | 420x60 | 350x50 |
| Substrate material | Si | Si | Si | Si | Si | Si | Si |
| Coating material | Au | Au/Pt | Au | Au | Au | Au | Au |
| Coating thickness [nm] | 30 | 30 | 30 | 30 | 30 | 30 | 30 |
| Line density [mm-1] | - | - | 600 | 800 | 1200 | - | - |
| Profile | - | - | lamellar | lamellar | lamellar | - | - |
The PGM monochromator has three diffraction gratings installed with different line densities, optimized for different energy ranges.
G1 (600 l/mm): 50 – 300 eV
G2 (800 l/mm): 200 – 1000 eV
G3 (1200 l/mm): 400 – 1500 eV
The beamline is equipped with an ionization cell, which enables to determine the energy resolution of light. Measurements carried out for various gases show that the actual resolution coincides with the predicted one (fig. 4).
Fig.4.Absorption spectra obtained in the ionization cell for Ar 2p3/2 → 4s oraz N21s → π* transitions.